When microwave ovens were first marketed for home kitchen use, the customary experience was for the food to warm and cook but not the container. Although the container often became warm, this was due to conduction of heat from the food and therefore the dish was limited to the heat of the food or some temperature less. This is still the situation today where "conventional" microwave cooking is employed utilizing cooking wares that are not heated by the oven but rather indirectly, by the food.
Notwithstanding the past and existing experiences, there was a recognition that the dish could also serve as a cooking surface to the food and therefore items have been developed which do not heat when subjected to microwave energy. This discovery was based on the phenomenon that some materials will absorb microwave energy, converting it to heat and these are said to be lossy as contrasted with transparent materials through which microwave energy passes without generation of heat. By making a cooking ware of lossy material, food can be cooked at the surface or exterior by conduction as well as by absorbing microwave energy.
One early approach was set forth in U.S. Pat. No. 2,830,162 wherein ferrite materials were included in the body of a cooking utensil. Ferrites absorb microwave energy to a temperature, the Curie temperature, beyond which power absorption decreases and heating does not continue. This property is well known as the Curie effect which was defined in the patent as the capacity of an element to resist additional conversion of radio frequency energy into heat after such element has been heated to a critical temperature constituting the Curie temperature for such material.
U.S. Pat. No. 3,701,872 also defines an implement for converting microwave energy into heat energy for use primarily in cooking. The implement includes a body transparent to microwave energy, preferably glass or ceramic, which contains a bed of resistive particles such as ferrites or carbon which will arc and form heat. A heat conducting element such as copper is interposed between the bed and a cooking surface to transfer the heat to the latter. The patent indicates that as the resistance of the particles varies, so does the heat, therefore, carbon can be utilized for refractory processes while ferrites are suited for household cooking.
U.S. Pat. No. 4,190,757 discloses yet another heating implement for microwave energy in the form of a disposable package. The package includes a lossy microwave energy absorber which becomes hot and transfers heat to the food in the package. The working layer of the package or heating body includes an upper structural member for support and heat resistance such as aluminum, copper, ceramic foil, cement or the like and a heating layer having a lossy substance capable of reaching a temperature above 100.degree. C. The latter substance is preferably a coating, likened to a thin layer of paint comprising a binder and a ferrite or similar material including powdered and granular Fe.sub.3 O.sub.4, other metallic oxides, carbides and dielectrics such as carbon.
U.S. Pat. No. 4,266,108 discloses a later development in microwave heating devices again relying on a lossy material in heat transfer relationship with a microwave reflective member which heats and cooks the food. The novelty is based upon selection of a magnetic ferrite containing material i.e., ferrites in pellet form or in a layer modified with agents such as glass frit, which is adhered to be reflective member with a bonding agent. The ferrite containing material of the invention has a specific volume reisistivity, expressed in ohm cm of greater than a value of Log R=(Tc/100)+2 where Tc is the Curie temperature of the ferrite material.
U.S. Pat. No. 4,450,334 dicloses a microwave pizza maker comprising a metallic pan and cover and a microwave transparent base. A layer of ferrite particles is attached to the underside of the pan to absorb energy and produce heat. The particles are preferably dispersed in a plastic layer, namely, high temperature silicone, 0.05 inches thick and bonded to the underside of the pan in any conventional manner.
Lastly, U.S. Pat. No. 4,454,403 discloses a heating apparatus which also employs a heat conductive layer to which is bonded a layer of lossy material. The latter is again described as a ferrite material dispersed in a high temperature plastic such as silicone.
Thus, as is evident, a varity of dishes and related wares have been based upon ferrite particles, carbon, metallic oxides and the like bonded to a transparent, conductive material or otherwise juxtaposed therewith. While specific lossy materials are specified, the material in which they have been dispersed, where dispersion is employed, has not been given as specific attention except for the two patents above which call for silicone rubber. It is believed that a novel browning ware can be provided which employs a unique plastic matrix and magnetite as the lossy material.